This invention relates to electrophoresis and to capillary electrophoresis used in conjunction with electrospray ionization mass spectrometry.
Electrophoresis is fundamentally the movement of charged particles within an applied electric field. Capillary electrophoresis (CE) is a known process. In capillary electrophoresis, a sample is injected at one end of the capillary. A detector is attached on the capillary at the other end of the capillary distant from the sample. A voltage is applied along the length of the capillary.
With the electric potential applied, two separate flow effects occur. The first of these flow effects is a gross sample flow effect. The sample moves as a mass into the capillary. The second of these flow effects is the electrophoretic flow. This causes the constituents of the sample having differing electric charge to move relative to the main stream of fluid within the capillary. The portions of the sample having differing electric charges are thereby separated in the capillary.
Different detectors may be used to analyze the sample after the separation has occurred. In a system that combines capillary electrophoresis with electrospray ionization (ESI), and mass spectrometry (MS), the output of the capillary is input to an electrospray assembly. The electrospray ionization is accomplished by placing a high voltage potential at the outlet of the separation capillary with respect to the capillary inlet to the mass spectrometer. The separation capillary also requires a high voltage potential placed between its inlet and outlet. The separated portions of the sample are dispersed by the electrospray into a fine aerosol as they exit the capillary. The droplets of the aerosol then are observed by mass spectrometry.
Capillary electrophoresis coupled with electrospray ionization and mass spectrometry is a relatively difficult procedure. The capillary must be mechanically connected to the rest of the system and positioned with respect to a detector. The capillaries are small and fragile, and the alignment process with the electrospray ionization assembly into the mass spectrometer may be difficult, time consuming, and may damage the capillary.
The system is further complicated by the need to cool the capillary. This cooling is required because the small capillary is subject to electrical resistance heating during the period of time electrophoresis potential voltage is applied. A small current under high voltage flowing in the capillary generates heat. The cooling is required to prevent damage to the capillary and to prevent variations in temperature during analysis of the sample from impacting the results of the analysis. Excess heat may cause diffusion of the separated portions of the sample that migrate through the capillary at different speeds. The heat and its resultant diffusion degrade separation and following classification result that is the purpose of using electrophoresis.
FIG. 2A describes a known CE-MS system using a CE power supply 220a that is not isolated. This configuration may also be considered a non-floating configuration, where the high voltage from CE power supply 220a used to create separation in the sample capillary 242a has the same ground as the ESI-MS power supply 254 that is used by mass spectrometer 250. Using non-isolated (non-floating) power supplies requires that the return path, which is the path that the current from the DC power supply output follows to return to the power supply input, for both separation and electrospray power supplies are ground referenced. This presents a problem in that the separation power supply return creates an electrical short circuit for the electrospray power supply. This is seen in FIG. 2A in the connection from electrospray (ES) High Voltage Output 226 through Conductive Fluid Capillary 244a to CE High Voltage (HV) return 224a. 
To alleviate this problem, one known approach is to have the high voltage return for the separation power supply disconnected. In FIG. 2A, this may be shown as disconnecting the direct path from conductive fluid vial 234a to CE Power Supply 220a. This changes the return path for CE Power Supply 220a, and then requires that current generated by the separation supply not only pass through the sample capillary 242a but also the electrospray assembly 252a, the mass spectrometer ESI-MS Power Supply 254, and the instrument chassis ground before finding its return to CE power supply 220a at CE HV return 224a. Although such a method provides the functionality for CE-ESI-MS, if the separation power supply requires current to return through a dedicated pin rather than chassis ground, a false current leakage reading may be induced. Furthermore, current measurements for the electrospray power supply will reflect the sum of both ESI and separation current. Because the separation current is typically one to two orders of magnitude greater than the electrospray current, the electrospray current cannot be determined for system diagnostic purposes.
Another alternative solution involves placing the electrospray needle or separation capillary outlet at ground potential. This may work with the operation of the CE-ESI-MS system but requires the inlet of the mass spectrometer to be at high voltage potentials. This complicates the design of the mass spectrometer.
Therefore, other alternative solutions to the problems presented by a non-isolated power supply may avoid the drawbacks of the known alternatives. Additionally, while systems and methods of using capillary electrophoresis with electrospray ionization and mass spectrometry are known, as described above, modifications to current systems as presented herein improve the ease of use, performance, and electrical functionality of these systems.